Stable linewidth narrowing of a coherent comb laser

ABSTRACT

A technique for narrowing a linewidth of a plurality of lines of a coherent comb laser (CCL) concurrently comprises providing a mode-locked semiconductor coherent comb laser (CCL) adapted to output of at least 4 mode-locked lines; tapping a fraction of a power from the CCL from the laser cavity to form a tapped beam; propagating the tapped beam to an attenuator to produce an attenuated beam; and reinserting the attenuated beam into the laser cavity, where the reinserted beam has a power less than 10% of a power of the tapped beam. The reinsertion allows the CCL to be operated to output the mode-locked lines, each with a linewidth of less than 80% of the original linewidth. By propagating the tapped and attenuated beams on a solid waveguide, and ensuring that the secondary cavity is polarization maintaining, improved stability of the linewidth narrowing is ensured.

FIELD OF THE INVENTION

The present invention relates in general to devices for stable narrowingof linewidths of quantum dot or dash mode-locked coherent comb lasers(CCLs), and in particular to a technique for concurrently narrowing aplurality of mode-locked modes using a self-injection external cavitywith high stability.

BACKGROUND OF THE INVENTION

Communication networks need to keep up with the growth of today'sInternet data traffic. The telecommunications industry needs newphotonics equipment to improve current optical networks and fordeployment in next generation optical networks. Semiconductor lasers areamong the most important generation components in opticaltelecommunication systems. Optical linewidth of semiconductor lasers isimportant because linewidth determines the laser's coherence length andphase noise. The maximum data rate in an optical fiber communicationslink is determined by the ratio of signal power to noise power as perthe Shannon-Hartley equation. Narrow linewidth is an essentialrequirement for lasers used in high data rate coherent communications,since phase noise impacts signal noise by the coherent detectionprocess. Even non-coherent modulation schemes can suffer from areduction in signal quality when phase noise is translated intoamplitude noise. Thus, lasers for modern optical communications systemsnow require linewidths of a few hundred KHz or less.

Unfortunately semiconductor lasers typically have linewidths on theorder of several to tens of MHz. Consequently, techniques for reducingoptical linewidth of semiconductor lasers have been of growingimportance since the move to coherent optical communications that hasbeen building over the last decade.

Accordingly, there has been a significant amount of interest in opticalcoherent comb lasers (CCLs) and their benefits as a source of multiplespectral lines (also known as “tones”) in coherent optical fibercommunications, because CCLs have been used to create the carrierfrequencies in dense wavelength division multiplexing (DWDM) opticalsystems with net data rates exceeding Terabit/s transmission rates andhigh spectral efficiency [1-3]. Different techniques have been used togenerate multi-wavelength lasers: modulator-based comb sources [3],spatial mode beating within a multimode fiber section [4], multi-cavityoscillation [5], comprising highly nonlinear fibers for spectralbroadening [6], or high-Q microresonators [7]. However, these techniqueseither require complex setups with discrete components, high pump powerswith delicate operating procedures, or they provide only a limitednumber of spectral carriers.

For practical systems, a compact, low-cost, energy-efficient CCL isdesired. Applicant developed nanostructured InAs/InP quantum dot (QD)multi-band (multi-colour) multiwavelength mode locked laser, and hasdemonstrated intra-band and inter-band mode-locking (U.S. Pat. No.7,991,023). Its use as a coherence comb laser (QD-CCL) over a largewavelength range covering C- or L-band has been demonstrated [8-18].Unlike uniform semiconductor layers in most telecommunication lasers, inthe QD CCL, light is emitted and amplified by millions of semiconductorQDs (typically less than 50 nm lateral diameter). Each QD acts like anisolated light source acting independently of its neighbours, and eachQD emits light at its own respective wavelength. By providing highefficiency QDs with a desired emission frequency distribution, the CCLis more stable and has much better performance compared to othermulti-wavelength lasers. Importantly, a single CCL has been shown tosimultaneously produce 50 or more separate lines at spatiallydistributed wavelengths over the telecommunications C-band or L-band. Toachieve these properties we have put considerable effort to design, growand fabricate InAs/InP QD gain materials and produce CCLs.

More recently Applicant has demonstrated CCLs with repetition rates from10 to 437 GHz and a total output power up to 50 mW, at room temperature[8-18]. Applicant has investigated relative intensity noises (RINs),phase noises, RF beating signals and other parameters of both filteredindividual channels and the whole CCL's output [17-18]. Unfortunately,the single filtered channels of QD CCLs generally exhibit strong phasenoise and broad optical linewidths, typically of the order of MHz[17-21]. As a consequence, wavelength-division multiplexing (WDM) datatransmission using these CCLs has been restricted to direct detectionschemes [22] or differential quadrature phase shift keying (DQPSK),which only uses relatively few (4) symbols. While these CCLs have highsymbol rates, their aggregate data rates (up to 504 Gbit/s [23]) arelimited by the symbol sets. Coherent transmission can use many more than4 symbols to achieve higher baud rates, where linewidth allows. The CCLsare not satisfactory for Tbit/s (and higher) coherence opticalnetworking systems.

Furthermore, other uses for CCLs, such as in high precision opticalmeasurement devices or high resolution spectral analysis, are limited bythis phase noise.

In order to improve net data transmission rates and spectral efficiencyin optical coherent communication systems, researchers have putsignificant efforts to simultaneously reduce optical linewidth of eachindividual channel of CCLs. For example, a feed-forward heterodynescheme has been used to simultaneously reduce the optical linewidth ofmany comb lines from mode-locked lasers [24-25]. Both [25], and [26] usea local oscillator (LO) and a Mach-Zender Modulator (MZM). The LOs havea narrow linewidth (narrower than the narrowest linewidth achieved bythe feedforward system). These references show the difficulty ofproducing a large set of comb lines (more than 20) simultaneouslynarrowed to a high degree (below a few hundred kHz), even when resortingto the relatively complex setups.

Prior art for reducing linewidth of single mode lasers are also known.For example, U.S. Pat. No. 8,804,787 to Coleman et al. claims aparticular arrangement for tapping a laser signal from a single modelaser cavity, attenuating the laser signal, and feeding the attenuated(−30 to −80 dB) laser signal back into the laser cavity, where the laserdriver provides sufficient drive stability so that a frequency variationof the laser is less than a free spectral range (FSR) of the secondarycavity. This patent specifically identifies as an unexpected result:“that an uncontrolled OPL[Optical Path Length] to the back reflectionprovided by the first branch provides significant spectral narrowing,which can be several orders of magnitude narrowing”. A reduction oflinewidth from 118 kHz to 2 kHz was demonstrated for a single wavelengthQD laser. “Polarization Maintaining (PM) fiber or non-PolarizationMaintaining SM fiber” can be used.

Recent papers [29,30] associated with a European Commission EC-FP7 BigPipes project demonstrate simultaneous linewidth narrowing of 60 linesin a Quantum Dash mode-locked laser diode using resonant feedback from asecondary cavity, without any LO. The secondary cavity is provided witha freespace optical setup from a backside facet of the mode-locked laserdiode that is barely disclosed. Freespace optical waveguides aretypically polarization maintaining. Stability of the linewidth is notdiscussed in any of the prior art references, including these recentpapers. Stability is particularly important for commercial deployment oflasers used in telecommunications applications. Given the highlyschematic description of the optical system provided in these papers, itis unclear what kind of stability could be provided with their system.Given that “the external cavity length is adjusted to be near a multiple[M] of the optical length of the laser”[30], and a known variably of thelaser optical length in operation, it is a safe assumption that thestability is poor outside of highly controlled lab settings. It shouldbe noted that a large OPL for the external cavity (which would bedesirable for a large linewidth reduction factor) will require thismultiple M to be large. However if M is large, a small variation in thelaser's effective OPL (S) will generate a difference MxS in the distanceof the reflector from the intended position. The ability to predict oradapt the OPL of the external cavity is not trivial, if possible, andboth the OPL of the external cavity and the attenuation have cumulativeeffects in terms of varying output, leading to a further source ofinstablility.

Accordingly there is a need for a technique for concurrently narrowinglinewidths of a plurality of mode-locked comb lines in a CCL, withoutrelying on a narrow linewidth LO and MZMs, without reducing a number oflines of the CCL, while retaining stability of the narrowed linewidth.Furthermore, there is a need for stably narrowing more linewidths of aCCL, to a greater extent, without complicated and expensive equipment tosetup and maintain.

SUMMARY OF THE INVENTION

Applicant has discovered a low-cost and efficient technique forsimultaneously narrowing linewidths of coherent comb lasers (CCLs) withimproved stability using a polarization maintaining fiber-basedsecondary cavity. The technique does not rely on narrower linewidthlocal oscillators (LOs), and Mach-Zender Modulators (MZMs), and can beachieved with less equipment and cost than such techniques. Thetechnique has demonstrably simultaneously reduced the optical linewidthof each of 39 individual channels of a 25 GHz QD CCLs from a few of MHzdown to less than 200 kHz, without reducing the number of lines, and hasstability far higher than what is possible with with long OPL freespaceoptics, and secondary cavities composed of non-polarization maintainingsingle mode fibre.

Accordingly a method for narrowing a linewidth of a coherent comb laser(CCL) is provided. The method involves: providing a mode-lockedsemiconductor coherent comb laser (CCL) with a laser cavity defined byan active gain material in a waveguide between two facets, the CCLadapted to output of at least 4 mode-locked lines, each with an originallinewidth of less than 100 MHz; tapping a fraction of a power from theCCL from the laser cavity to form a tapped beam; propagating the tappedbeam to an attenuator to produce an attenuated beam and propagating theattenuated beam back to the laser cavity, on a solid waveguide; andreinserting the attenuated beam into the laser cavity, where thereinserted beam has a power less than 10% of a power of the tapped beam.The reinsertion allows the CCL to be operated to output the mode-lockedlines, each with a linewidth of less than 80% of the original linewidth,and an optical path between tapping and reinsertion is polarizationmaintaining.

The mode-locked semiconductor CCL provided, preferably: is adapted tooutput at least 10 mode-locked lines; is adapted to output at least 4mode-locked lines with original linewidths between 10 and 80 MHz; isadapted to output at least 10 mode-locked lines in an optical networkingtelecommunications band; is electrically pumped; is a ridge waveguidelaser with edge facets forming a Fabry-Perot cavity; is one of: a smalledge-emitting laser, an external cavity laser, a monolithic(internal-cavity) laser, a diode bar laser, a stacked diode bar laser, asurface-emitting laser (VCSEL), such as an optically pumpedsurface-emitting external-cavity semiconductor laser (VECSEL), or aquantum cascade laser; or has an active gain material comprising quantumwells, dots, dashes or rods formed of GaAs, AlGaAs, InGaAs, InAs,GaInNAs, GaN, GaP, InGaP, InP, GaInP, or a combination thereof. Morespecifically, the CCL preferably is adapted to output at least 10mode-locked lines with original linewidths between 10 and 80 MHz, orbetween 1 and 30 MHz; is adapted to output at least 25 mode-locked linesin an optical networking telecommunications “C” band; is electricallypumped, controlled by a low noise laser driver, and temperaturecontrolled; or has an active gain material comprising quantum dots,and/or dashes formed of GaAs, AlGaAs, InGaAs, InAs, GaInNAs, GaN, GaP,InGaP, InP, GaInP or a combination thereof.

An optical path length of the secondary cavity is preferably between 5and 50 m, and the attenuation level is preferably between 15 and 60 dB.

Tapping the CCL preferably comprises: collecting output of a backsidefacet of the CCL, or providing a coupler to tap a fraction of an outputof the CCL.

Reinserting the attenuated beam preferably comprises reinjecting theattenuated beam into the laser cavity via the backside facet, or thecoupler.

Propagating the tapped beam to an attenuator preferably comprises:coupling the tapped beam from a bidirectional waveguide path to aunidirectional waveguide circuit including the attenuator; coupling thetapped beam from a bidirectional waveguide path, which includes theattenuator, to a unidirectional waveguide circuit; providing theattenuator on a bidirectional waveguide path that includes a reflector;or providing a partial reflector on the bidirectional waveguide paththat serves to both attenuate and reflect the tapped beam.

Coupling the tapped beam is preferably provided by an opticalcirculator.

The attenuator is preferably a variable optical attenuator.

The attenuator preferably has an attenuation range of at least 10 dB;avoids creating spurious reflections; attenuates each of the lines tosomewhat the same degree; and does not vary an OPL of the secondarycavity while changing the degree of attenuation. The attenuatorpreferably controls light transmission by an aperture variation, withpartial occlusion of the beam.

The solid waveguide of the optical path between tapping and reinsertionis preferably provided by single mode optical fibres, a microphotonicchip, a photonic crystal arrangement, or an integrated optical system.

One of the mode-locked lines output preferably has a stability such thatover a one hour period, the linewidth does not vary by more than 100kHz.

Also accordingly, a narrow linewidth multi-wavelength laser (MWL) isprovided, comprising: a mode-locked semiconductor coherent comb laser(CCL) with a laser cavity defined by an active gain material in awaveguide between two facets, the CCL adapted to output of at least 4mode-locked lines, each with an original linewidth of less than 100 MHz;and a secondary cavity coupled to the laser cavity for tapping a beam ofthe CCL and propagating the tapped beam to an attenuator and reinsertingthe attenuated beam into the cavity at a power less than 10% of a powerof the tapped beam, the secondary cavity consisting of polarizationmaintaining solid waveguides between polarization maintainingcomponents. A linewidth of each of the at least 4 lines is reduced inproportion to a difference in optical path length between the feedbackcavity and the laser cavity.

Preferably the CCL: is adapted to output at least 10 mode-locked lines;is adapted to output at least 4 mode-locked lines with originallinewidths between 10 and 80 MHz; is adapted to output at least 10mode-locked lines in an optical networking telecommunications band; iselectrically pumped; is a ridge waveguide laser with edge facets forminga Fabry-Perot cavity; is one of: a small edge-emitting laser, anexternal cavity laser, a monolithic (internal-cavity) laser, a diode barlaser, a stacked diode bar laser, a surface-emitting laser (VCSEL), suchas an optically pumped surface-emitting external-cavity semiconductorlaser (VECSEL), or a quantum cascade laser; or has an active gainmaterial comprising quantum wells, dots, dashes or rods formed of GaAs,AlGaAs, InGaAs, InAs, GaInNAs, GaN, GaP, InGaP, InP, GaInP, or acombination thereof. More specifically, the CCL preferably: is adaptedto output at least 10 mode-locked lines with original linewidths between10 and 80 MHz, or between 1 and 30 MHz; is adapted to output at least 25mode-locked lines in an optical networking telecommunications “C” band;is electrically pumped, controlled by a low noise laser driver, andtemperature controlled; or has an active gain material comprisingquantum dots, and/or dashes formed of GaAs, AlGaAs, InGaAs, InAs,GaInNAs, GaN, GaP, InGaP, InP, GaInP or a combination thereof.

An optical path length of the secondary cavity is preferably between 5and 50 m, and the attenuation level is between 15 and 60 dB.

The secondary cavity preferably comprises an optical coupling from oneof a backside facet of the CCL, and/or a tap of an output of the CCL viawhich the beam is tapped and/or reinserted. The secondary cavitypreferably comprises: a bidirectional waveguide path coupled to aunidirectional waveguide circuit including the attenuator; abidirectional waveguide path, including the attenuator, coupled to aunidirectional waveguide circuit; a bidirectional waveguide path thatincludes a reflector; or a partial reflector on the bidirectionalwaveguide path that serves to both attenuate and reflect the tappedbeam.

The coupling of the tapped beam may be provided by an opticalcirculator.

The attenuator may be a variable optical attenuator with an attenuationrange of at least 10 dB, provisioned to avoid creating spuriousreflections, to attenuate each of the lines to somewhat the same degree,and to not vary an OPL of the secondary cavity while changing the degreeof attenuation. The attenuator preferably controls light transmission byan aperture variation, with partial occlusion of the beam.

The secondary cavity preferably comprises an optical path betweentapping and reinsertion provided by: single mode optical fibres; afree-space optical system; a microphotonic chip; a photonic crystalarrangement; or an integrated optical system.

One of the at least 4 lines preferably has a stability such that over aone hour period, the linewidth does not vary by more than 100 kHz.

Further features of the invention will be described or will becomeapparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, embodimentsthereof will now be described in detail by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a flowchart illustrating principal steps of a method inaccordance with an embodiment of the present invention;

FIG. 2a is a schematic illustration of apparatus accordance with anembodiment of the present invention;

FIG. 2b is a schematic illustration of apparatus accordance with anotherembodiment of the present invention;

FIG. 2c is a schematic illustration of apparatus accordance with anotherembodiment of the present invention;

FIG. 2d is a schematic illustration of apparatus accordance with anotherembodiment of the present invention;

FIG. 2e is a schematic illustration of apparatus accordance with anotherembodiment of the present invention;

FIG. 2f is a schematic illustration of apparatus accordance with anotherembodiment of the present invention;

FIG. 3A pertains to a CCL used to demonstrate the present invention, inparticular 3A is a schematic illustration of a mode-locked InAs/InPQuantum Dash CCL used for the demonstration of the present inventionwith a micrograph inset;

FIG. 3B also pertains to a CCL used to demonstrate the presentinvention: 3B shows laser output spectrum showing the characteristiccomb output of the same CCL;

FIG. 4A is a graph, in conjunction with 4 b, comparing single lineoptical noise spectra for optimized driven CCL in comparison with thesame CCL with secondary cavity self-feedback respectively for the 1^(st)and 15^(th) lines of the CCL;

FIG. 4B is also a graph, in conjunction with 4A, comparing single lineoptical noise spectra for optimized driven CCL in comparison with thesame CCL with secondary cavity self-feedback respectively for the 1^(st)and 15^(th) lines of the CCL;

FIG. 5 is a linear graph showing linewidth of a CCL with optimizeddriving without and with a secondary cavity self-feedback;

FIG. 6 is a graph showing normalized RF beating signal spectra foroptimized driven CCL in comparison with the same CCL with secondarycavity self-feedback; and

FIG. 7 is a graph showing optical linewidth as a function of time of anindividual channel from a self-locked 25 GHz QD CCL comparing apolarization maintaining secondary cavity with a non-polarizationmaintaining secondary cavity.

DESCRIPTION OF PREFERRED EMBODIMENTS

Herein a cost-effective technique for reducing linewidth of a coherentcomb laser (CCL) is described that provides higher stability of thenarrowed lines. The technique avoids use of narrow linewidth LocalOscillators and Mach-Zender Modulators, and does not reduce a bandwidth,or number of lines of the CCL, and simultaneously reduces linewidth of anumber of mode-locked lines. The technique uses polarization maintainingsolid waveguide between at least a multimode laser and attenuator toproduce a secondary cavity that is substantially polarizationmaintaining.

FIG. 1 is a schematic illustration of a method for reducing linewidth ofmode-locked lines of a CCL, in accordance with the present invention.The method begins at step 10 by providing a CCL that has at least a few,such as 4 mode-locked emission lines (also referred to as emissionfrequencies, or wavelengths). For telecommunications purposes,preferably the CCL has as many lines as possible within thetelecommunication bands, and these are preferably evenly spaced apart.The CCL is preferably electrically pumped, and is a semiconductor laser.The CCL may be a ridge waveguide laser with edge facets forming aFabry-Perot cavity. The type of semiconductor laser may be: a smalledge-emitting laser; an external cavity laser; a monolithic(internal-cavity) laser; a diode bar laser; a stacked diode bar laser; asurface-emitting laser (VCSEL); such as an optically pumpedsurface-emitting external-cavity semiconductor laser (VECSEL); or aquantum cascade laser. The CCL may have an active material of quantumwells, dots, dashes or rods formed of GaAs, AlGaAs, InGaAs, InAs,GaInNAs, GaN, GaP, InGaP, InP, or GaInP, and more preferably quantumdots or dashes.

A set of the lines of the CCL are mode-locked if driven by a suitablelow noise laser driver. Linewidths of these lines are principallydetermined by phase noise: a variation of the instantaneous linefrequency over time. The property of mode-locked lines is that thisphase noise varies similarly as a function of time at each of the lines,but the frequency variations at the different lines may vary inamplitude, thus it is common for linewidths to vary gradually, oftenmonotonically, across the spectrum of a mode-locked laser. A range ofthese linewidths, are important indicators of how much improvement tothe modelocking, and to the linewidth (or phase noise, or frequencyvariation), the present invention can produce, and what Optical PathLength (OPL) the secondary cavity should have, as explained hereinbelow.

At step 14, a CCL output is tapped uniformly across at least the gainspectrum of the mode-locked lines to form a beam. This may be done byeither facets of the CCL, or by a beam splitter on the CCL output. TheCCL may be a symmetric laser, with identical facet coatings on both endsof the FP cavity, and the tapped beam can be drawn from either laserfacet. The power of the tapped beam, relative to the CCL output may bedetermined by a transmittance of the facet by which the tapped beam isdrawn, or by a coupler. Care is taken to ensure that the tapped powerforms a beam without reflecting power back into the cavity at differentdistances from the facet. The tapped beam is transmitted through apolarization maintaining (PM) single mode fiber (SMF) as shown in thesystem embodiments of the invention herein, although a microphotonicchip with suitable high quality coupling and optical path length couldalternatively provide an advantageously integrated optical arrangement.Both PM-SMF and PM microphotonic chips are solid waveguide technologythat provide a desired optical path length for the secondary cavity.While it will be appreciated that some part of the secondary cavity maybe provided by freespace optics, such as within a preferred variableoptical attenuator, or within certain optical circulators, the wholesecondary cavity remains PM, and the waveguides are nearly exclusivelysolid. It should be noted that photonic crystal arrangements may beprovided in the PM-SMF or microphotonic chip.

The tapped beam is attenuated in transit through the secondary cavity(step 14) exclusive of the CCL's (primary or FP) cavity. Herein thesecondary cavity subsumes the FP cavity and further extends from tap toreinsertion. The attenuation preferably includes at least one controlledattenuator, that allows for varying a degree of attenuation. Thecontrolled attenuator preferably operates in a manner that does notreflect the beam (i.e. avoids creating spurious reflections); attenuateseach frequency to somewhat the same degree; and does not vary an OPL ofthe secondary cavity while changing the degree of attenuation. Anaperture-based variable optical attenuator may accomplish thiseffectively. However, a well-controlled optical path with a fixedattenuation at the right level and the correct optical path length canequally function. Also, as can be gleaned by the equation hereinbelow,control over OPL of the secondary cavity is an equivalent for controlover attenuation (in the limiting case of optimized feed-back), althoughprecise control over OPL is more technically challenging thanattenuation.

In step 16, the attenuated beam is reinserted into the laser cavity,with at least a decimated power (i.e. at most 1/10th of the powertapped). Herein, all physical ranges and half-ranges for parameters areintended to equally support every subrange thereof. The reinsertion maybe via the first or second laser facet.

To design such a system for a given CCL, one must choose the OPL(generally 1-1000 m; more preferably 5-300 m; more preferably 10 to 50m) and the attenuation level (generally 10-80 dB, more preferably 30-60dB) of the secondary cavity.

Specifically, selection of L_(ext), the OPL of the secondary cavity, canbe made with a CCL once the following parameters of the CCL are known:L_(cav), the OPL of the effective laser cavity; r_(ext), an amplitudereflection coefficient of the external cavity (square root of laserpower reflection coefficient across the secondary cavity); r_(cav), anamplitude reflection coefficient of the laser cavity where it joins thesecondary cavity; and α, a linewidth enhancement (Henry) factor. It iscommon knowledge how to measure these parameters. Assuming resonance,the equation relating F, the linewidth narrowing ratio (linewidthoptimized with the secondary cavity/linewidth optimized without), withL_(ext) the OPL of the secondary cavity, is:

$F = \lbrack {1 + {{\frac{L_{ext}}{L_{cav}}.\frac{r_{ext}}{r_{cav}}.\sqrt{1 + \alpha^{2}}}( {1 - r_{cav}^{2}} )}} \rbrack^{- 2}$

This equation, though simplified for optimized conditions, allows forestimation of the maximum linewidth narrowing factor. The problem ofextending the OPL and attenuation of the secondary cavity across alargest range of lines is non-trivial and depends on many factors thatare known to those of skill in the art. In general, an RF beating signalspectrum of the CCL is observed, and if its linewidth is less than a fewhundred kHz it is sufficiently mode-locked. Furthermore the opticalphase noise of each (or a representative number of) individual lines isassayed to determine the total phase noise. If the optical phase noiseof each line is less than 100 MHz, and a variation in optical phasenoise across the lines is less than 100 times, the present invention isexpected to narrow linewidth by a factor of at least 20%. In general thelower the optical phase noise of the lines, the higher the OPL can bechosen, and the higher the gain factor achievable, subject to theability to achieve resonant conditions.

FIG. 2a is a schematic illustration of a first embodiment of the presentinvention. The first embodiment includes a secondary cavity defined at abackside of a CCL 20. Herein frontside of CCL 20 is an end of the laserfrom which the output is emitted, and backside is opposite thefrontside. Herein like features are identified by like referencenumerals, and their descriptions are not repeated in each embodiment ofthe invention, unless to point out a different aspect of the invention.

The CCL 20 is a semiconductor laser controlled and electrically excitedby a laser driver 21, and a thermoelectric cooler (TEC) 22. It will beappreciated that the laser driver 21 is optimized for controlling laseroutput of the CCL 20 with the secondary cavity feedback. The backside iscoupled to a first segment of polarization maintaining single mode fibre(PM-SMF) 23 a, in a manner well known in the art for avoiding backreflections. The first segment is coupled to port 1 of a polarizationmaintaining optical circulator (PM-OC) 24. The signal from port 1 isemitted from port 2 of the PM-OC 24 coupled to a second segment of thePM-SMF 23 b. The second segment 22 b is coupled to a variable opticalattenuator (VOA) 25 which attenuates the beam, and forwards theattenuated beam along a third segment of the PM-SMF 23 c, which couplesto port 3 of the PM-OC 24. One advantage of using a PM-OC is that backreflections to the first segment 23 a are essentially precluded. Anyreflections entering port 2, whether back reflected from the VOA 25, orcycling through port 3, will substantially exit port 3 in an indefinite,highly attenuated, loop. The attenuated beam received at port 3 isoutput to port 1 for reinsertion into the CCL cavity, via the firstsegment 23 a. A fixed optical path length of the secondary cavity isprovided with a spool 26. An advantage of the location of the spool 26on the second or third segments is that any coupling (Fresnel)reflection losses are not propagated back to the CCL cavity except viathe port 3-port 1 path. An advantage of locating the spool 26 in thefirst segment, assuming no coupling reflection, is that only half thespool length is needed to provide the OPL.

FIG. 2b is a schematic illustration of a second embodiment of thepresent invention. The second embodiment differs from the firstembodiment in that the PM-OC 24 is replaced with a PM coupler 27, and anisolator 28 is placed in the secondary cavity. The PM coupler 27receives the tapped beam on segment 23 a, and may couple with 90%efficiency to segment 23 b, or may be a balanced 50-50 1:2splitter/combiner. Preferably there are no back-reflections. Even asmall (0.01%) reflection risks multiplexing multiple OPL feedbacksignals to the CCL, may spoil the feedback and make the secondary cavityuncontrollable. To the extent that the tapped beam is divided and goesinto a back side of the isolator 28, the isolator 28 serves as asecondary attenuator. It will be appreciated that the isolator 28 isdescribed as preventing light coupling from segment 23 d to 23 a, but itwould be equivalent if the isolator operated in the opposite direction.The isolator 28 may be a multi-stage isolator with a high isolationfactor. Alternatively, to provide equivalent unidirectivity, a pluralityof single stage isolators may be distributed before and after the spool26, and/or before and after the VOA 25.

FIG. 2c is a variant of the first embodiment, with an added PM coupler27 is used to permit the secondary cavity to branch from a laser outputon the frontside of the CCL cavity. The coupler 27 is preferably a 90:10coupler that sends 90% of the light to an isolator 28 for emission ofthe laser. The isolator 28, preferably a multi-stage isolator, isprovided on the frontside to prevent back reflections from the laser asused, from affecting laser stability, as is conventional on such lasers.The PM-OC 24 of FIG. 2c is a 4-port circulator, in which any beam thatis not transmitted from port a to port a+1, exits at port a+2. Byskipping port 3 in this setup, any component of the tapped beam that isnot sent to the VOA 25, is withdrawn via port 3. Any attenuated beamthat is not coupled back to port 1, is re-attenuated at the VOA 25 andwill have negligible effect on the CCL 20 with twice the attenuation.

While FIG. 2c subsumes essentially the embodiment of FIG. 2a , the ringcould alternatively be provided as per the embodiment of FIG. 2b , if a1 by 3 coupler is used instead of the 1 by 2 coupler, or two 1 by 2couplers are used in series from the laser output.

Furthermore, as shown in FIG. 2d , the tapped beam may be from thefrontside of the CCL 20, and the reinsertion can be into the backside ofthe CCL 20. By switching a direction of the isolator, the opposite isprovided, and is equally feasible.

FIG. 2e schematically illustrates a full duplex embodiment of thesecondary cavity. No isolator or circulator is used, but retroreflectionis provided by a mirror 29. The mirror 29 may be a partially reflective,or highly reflective coated end of the fibre with suitable attention topreventing other light from entering the coated end.

FIG. 2f is similar to FIG. 2e but uses a partial reflector 30 with aknown and controlled fractional reflectivity and fractionaltransmissivity. The transmitted beam is lost to the secondary cavity,and isolator 28 prevents any further reflections from entrance into thesystem. This set up requires a controlled but fixed OPL opposed to theVOA 25, to induce retroreflection, which has the disadvantage of notpermitting any reconfiguration after initial set up and calibration ofthe CCL, other than what can be achieved by varying the laser driver21's injection. After this retroreflection, an isolator 28 is providedto prevent further reflections from entering into the secondary cavity.

In the previous embodiments, the features used in one embodiment cangenerally be added or replaced with those of other embodiments withoutdeparting from the intended range of embodiments illustrative of thepresent invention.

EXAMPLES

FIG. 3A is a schematic illustration of the InAs/InP Quantum Dash CCLused for the demonstration of the linewidth narrowing with a secondarycavity. The InAs/InP QD CCL was grown by chemical beam epitaxy (CBE) onexactly (100) oriented n-type InP substrates. The undoped active regionof the QD sample consisted of five stacked layers of InAs QDs withIn0.816Ga0.184As0.392P0.608 (1.15Q) barriers. The QDs could be tuned tooperate in the C- or L-band using a QD double cap growth procedure and aGaAs sublayer [27-28]. In the double cap process the QDs are partiallycapped with a thin layer of InP, followed by a 30 second growthinterruption and then complete capping with the 1.15Q barrier material.A thickness of the partial cap controls a height of the QDs, and hencetheir emission wavelength, and is also narrows the height distributionof the QDs, resulting in a narrower 3-dB gain spectrum. The thin GaAssublayer promotes dash rather than dot growth. This active layer wasembedded in a 355 nm thick 1.15Q waveguiding core, providing bothcarrier and optical confinement. The waveguiding core was surrounded byp-doped (top) and n-doped (bottom) layers of InP and capped with aheavily doped thin InGaAs layer to facilitate the fabrication of lowresistance Ohmic contacts.

This sample was fabricated into a single lateral mode ridge waveguidelaser with a ridge width of 1.8 μm, and then cleaved to form a F-P lasercavity for the CCL. A laser cavity with length 1693 μm was produced forthe CCL. The output of this CCL was coupled to an anti-reflection (AR)coated lensed fiber followed by a two-stage C-band optical isolator toreduce any back-reflection to the QD CCL. The laser was driven with a DCinjection current using a low noise laser driver (ILX Lightwave modelLDX-3620B), and tested with a heat sink maintained at 20° C. using athermoelectric cooler (Melcor).

The performance of the QD CCL was characterized using an opticalspectrum analyzer (Anritsu MS9740A), a 50 GHz (max.) PXA signal analyzer(Keysight Technologies Model N9030A), a 45 GHz IR photodetector (NewFocus Model-1014), an optical autocorrelator (Femtochrome Research Inc.FR-103HS), a delayed self-heterodyne interferometer (Advantest Q7332 andR3361A), an OE4000 automated laser linewidth/phase noise measurementsystem (OEWaves Inc.) and power meters (Newport 840, ILX LightwaveFPM-8210H and OMM-6810B). FIG. 3B shows 58 lines output by the laserwith the optical spectrum analyzer.

L-I-V curves were measured for the CCL, and the lasing threshold currentwas found to be 48 mA, with a slope efficiency of 0.13 mW/mA. Thefollowing properties were obtained for the CCL in its original state:active length: 1693 μm; frequency spacing: 25 GHz; injection current:380 mA; temperature: 20° C.; center wavelength: 1537.7 nm; 3-dBbandwidth: ˜10.46 nm; and channels with the optical signal-to-noiseratio (OSNR) of more than 35 dB:at least 39. The laser's seriesresistance is 1.46 Ohm. The optical average output power measured by alarge area detector is 42 mW in these conditions. The optical linewidthof each individual channel is from ˜1 MHz to 4.5 MHz between 1542.92 nmto 1532.46 nm over the 53 channels, as graphically shown in FIG. 4.

While this CCL is excellent—all laser channels with excellent OSNR arevery stable because of highly inhomogeneous QD gain broadening due tostatistically distributed sizes/geometries, composition and environmentof self-assembled QDs—experimental results have clearly shown that theoptical linewidth of the single filtered channels of 1-4.5 MHz is notgood enough for: terabit/s (or better) coherence optical networkingsystems; high precision optical measurement; or high resolution spectralanalysis. In order to narrow the optical linewidth of every individualchannel of the QD CCLs, the simple external cavity, self-feedback systemwas invented.

The secondary cavity substantially as shown in FIG. 2c was used todemonstrate this invention, but with a 3 port PM OC. The frontside laseroutput of the QD MWL was optically coupled to an anti-reflection (AR)coated lensed polarization-maintaining (PM) single-mode fiber (SMF),followed by a two-stage optical isolator to prevent reflection back tolaser cavity from the measurement system. Naturally the TE alignment ofthe laser output was aligned with the polarization angle of the PM-SMF.The backside facet of the QD CCL was optically coupled to anotherAR-coated lensed PM-SMF (23 a) connected with port 1 of the PM opticalcirculator (OC) (Lightstar Inc. Model: PM0C-1550-B-900-5-0-0.8 5.5×35 mmFC/APC X3). The VOA 25 was a PM VOA based on a mechanical aperture toocclude part of the tapped beam without inducing any optical path changewhile adjusting attenuation. The VOA 25 is (Lightstar Inc. Model:PMVOA-1550-I-900-5-0-0.8 26×18×8 FC/APC X2) is adapted to attenuate1%-99% of a power of the tapped beam.

The secondary ring optical PM fiber cavity thus produced a self-injectedoptical feedback cavity that is weakly coupled to the laser cavity fortapping a fraction of a power via a backside of the QD CCL andreinserting it with an estimated power of 10⁻³ to 10⁻⁵ of that of thetapped beam.

The schematic in FIG. 2c shows a spool 26 of a fixed length chosen toprovide an OPL for the secondary cavity, however, the three lengths ofPM-SMF extended from the PM-OC, along with the PM OC between the CCL andcoupler 27 were sufficient to produce the 11 m OPL required. Thus thespool 26 was distributed in this instance. The secondary ring PM fibercavity had an OPL of a few thousand times longer than the ˜1.7 mm cavitylength of the QD CCL was found to produce a very strong linewidthnarrowing function of an ultra-narrow filter. In this case the laserpower fed back via the secondary cavity is used to improve laserlocking, bringing about significant narrowing of the laser emittingspectrum.

After the CCL was coupled to the secondary cavity, it's powercharacteristics were altered, and the ultra-low noise driver wasre-optimized for the new laser characteristics. In a manner known in theart the OPL of the laser cavity was varied to rematch a phase of thecavity for the secondary cavity feedback.

FIGS. 4A,B are both graphical comparisons of single line's phase noisefor an optimized driven laser with and without the secondary cavityself-feedback. FIG. 4A shows the first line, which is around 1545 nm(see FIG. 3B) and already has a narrowest linewidth without secondarycavity self-feedback (labeled C #1 OLw). This noise profile shows amany-peaked, generally higher amplitude phase noise below 1 MHz, with ageneral decrease in amplitude as frequency increases, but after about 1MHz, the phase noise amplitude flattens out. In contrast the phase noiseof the same channel with secondary cavity self-feedback (labelled C #1OLw/o) has a substantially worse noise profile below 1 MHz, but animproved noise profile above 1 MHz that more than offsets the lossesbelow 1 MHz. It will be appreciated by those of ordinary skill, that lowfrequency phase noise is readily compensated by low noise drivers ofcurrent semiconductor lasers. It therefore is clear from this graphicalcomparison that the 1s^(t) channel will be improved by a suitabledriver.

FIG. 4B shows again that for the 15^(th) channel, that above 1 MHz, thenoise amplitude of the line with the secondary cavity self-feedback (C#15 OLw/o) is appreciably lower for a typical line.

FIG. 5 is a graphical representation of linewidths of measured lines ofthe CCL with optimized drivers, showing lines without the secondarycavity self-feedback having linewidths of 0.9-4.5 MHz corresponding withlines with the secondary cavity self-feedback with linewidths of lessthan 200 kHz.

Table 1 lists measured channel numbers (C #), wavelength (in nm),optical linewidth without secondary cavity self-feedback (0Lw/o) in MHz,optical linewidth with secondary cavity self-feedback (OLw), in MHz, andthe reduction ratio (Ratio). This is the data graphed in FIG. 5. Itclearly shows the improvement to mode-locking produced by the secondarycavity self-feedback which has its greatest effect for lowest linenumbers.

C# Wavelength OLw/o OLw Ratio 1 1545.14 0.92 0.012 76.67 2 1544.94 0.930.013 71.54 3 1544.741 0.95 0.015 63.33 4 1544.54 0.97 0.017 57.06 51544.3395 1 0.024 41.67 6 1544.139 1.1 0.03 36.67 7 1543.94 1.16 0.03533.14 8 1543.741 1.25 0.044 28.41 11 1543.142 1.52 0.056 27.14 151542.343 2 0.08 25.00 19 1541.55 2.34 0.095 24.63 23 1540.75 2.89 0.12123.88 27 1539.951 3.21 0.136 23.60 31 1539.151 3.79 0.162 23.40 351538.352 4.22 0.182 23.19 39 1537.552 4.51 0.198 22.78

Linewidths of individual channels of the CCL as a function ofwavelength, for both the original CCL, and the CCL with the secondarycavity show the remarkable reduction in linewidth, especially for higherwavelength lines. Reduction of the laser linewidths is dramatic: forexample the line near 1538.5 nm shows a reduction factor of about 35(the linewidth with secondary cavity is about 3% the linewidth without).All of the lines from 1537.5-1545 originally had linewidths above about0.9 MHz, are now well less than 200 kHz, varying from about 1.2% to 4.4%of the original (without secondary cavity) feedback.

Normalized RF beating signal spectra, with and without self-injectionfeedback, is further plotted in FIG. 6. The RF beating signal spectraparticularly illustrate the non-common mode noise properties of thelines, in that any co-variation of the lines are not represented, as thelines beat against each other, (if they co-vary, this variation isfiltered out). The RF beating signals show a substantially narrowerpeak, and lower baseline (˜−50 dB) with the self-injection feedback, asopposed to the original output of the CCL, which has a baseline of about−27 dB. The RF beating signals with small RF full width at half maximum(fwhm: ˜300 Hz), clearly shows that the phase fluctuations of theirlongitudinal modes are synchronized and correlated, as expected in aphase-mode-locked laser.

While the foregoing improvements to linewidth are in line with prior artlinewidth improvements using secondary cavity self-feedback, the presentinvention provides that these linewidths are highly stable. High speedmeasurement of phase noise using heterodyne detectors can show similarphase noise improvements, without purporting to provide stability of thelinewidth.

In order to show the variability, FIG. 7 graphs 8 measurements usinghomodyne detection (OEWaves Inc. OE 4000 TM). Each measurement produceda scan as shown in FIG. 4A,B over a period of about 15 minutes. The samesecondary cavity and CCL were used, except for the components beingreplaced with polarization maintaining solid waveguides (bottom plotidentified with squares) vs. non-polarization maintaining solidwaveguides (top plot triangles). As mentioned above, if a heterodyne orself-homodyne measurement is performed, a very small sample time isrequired, and a measured linewidth approaching the square plot can beobtained, however subsequent measurements, even within minutes, will notshow the same value.

In general, the experiments were performed for both PM and non-PM solidwaveguide secondary cavities with monitored feedback power to keep thesame intensity (some measurements involved realigning the currentunpackaged CCL prior to measurement). The operation conditions are 330mA and 20° C., as before. The tested individual channel's wavelength is1540 nm.

The PM solid waveguide secondary cavity clearly shows both a far lowerlinewidth, and far less variation. The specific 8 data points providedfor both PM solid waveguide secondary cavities show a range of 0.9-8.3kHz difference between measurements separated by 1 hour (start time tostart time). The mean difference between measurements is 4.9 kHz, andthe standard deviation of the 8 values is 5.9 kHz. The non-PM datapoints show a range of 350-1395 kHz between successive measurements. Themean difference is 820 kHz, and the standard deviation of the 8 valuesis 570 kHz. Accordingly, it is observed that using PM solid waveguidesecondary cavity self-feedback, a linewidth of the laser cavity can beimproved by at least 20%, and have a stability such that over one hourthe linewidth does not vary by more than 100 kHz, more preferably bymore than 80 kHz, more preferably by more than 40 kHz, 20 kHz, or about5 kHz on average.

Applicant notes that stability over larger periods have been performed,and the PM-SMF secondary cavity self-feedback has been shown to be verystable over even longer periods.

REFERENCES

-   1. V. Ataie, E. Temprana, L. Liu, E. Myslivets, B. P.-P. Kuo, N.    Alic, and S. Radic, “Flex-grid compatible ultrawide frequency comb    source for 31.8 Tb/s coherent transmission of 1520 UDWDM channels,”    In the Proceedings of The Optical Fiber Communication Conference    2014, Postdeadline Paper, Th5B.7;-   2. Joerg Pfeifle, Victor Brasch, Matthias Lauermann, Yimin Yu,    Daniel Wegner, Tobias Herr,Klaus Hartinger, Philipp Schindler,    Jingshi Li, David Hillerkuss, Rene Schmogrow, Claudius Weimann,    Ronald Holzwarth, Wolfgang Freude, Juerg Leuthold, Tobias J.    Kippenberg and Christian Koos, “Coherent terabit communications with    microresonator Kerr frequency combs,” Nature Photonics, Vol. 8,    375-380 (2014);-   3. C. Weimann, P. C. Schindler, R. Palmer, S. Wolf, D. Bekele, D.    Korn, J. Pfeifle, S. Koeber, R. Schmogrow, L. Alloatti, D. Elder, H.    Yu, W. Bogaerts, L. R. Dalton, W. Freude, J. Leuthold, and C. Koos,    “Silicon-organic hybrid (SOH) frequency comb sources for terabit/s    data transmission,” Optics Express, vol. 22, 3629-3637 (2014);-   4. Z. G. Lu, F. G. Sun, G. Z. Xiao, P. Lin, and P. Zhao, “High-power    multiwavelength Er3+−Yb3+ codoped double-cladding fiber ring laser,”    IEEE Photon., Technol. Lett., Vol. 17, (9), pp.1821-1823, 2005;-   5. Z. G. Lu, and C. P. Grover, “A widely tunable narrow-linewidth    triple-wavelength erbium-doped fiber ring laser,” IEEE Photon.    Technol. Lett., Vol. 17, (1), pp.22-24, 2005;-   6. B. J. Puttnam, R. S. Luis, W. Klaus, J. Sakaguchi, J.-M. Delgado    Mendinueta, Y. Awaji, N. Wada, Yoshiaki Tamura, Tetsuya Hayashi,    Masaaki Hirano and J. Marciante, “2.15 Pb/s Transmission Using a 22    Core Homogeneous Single-Mode Multi-Core Fiber and Wideband Optical    Comb,” In the Proceedings of the 2015 European Conference on Optical    Communication (ECOC 2015), Postdeadline Paper 3.1;-   7. J. Pfeifle, A. Kordts, P. Marin, M. Karpov, M. Pfeiffer, V.    Brasch, R. Osenberger, J. Kemal, S. Wolf, W. Freude, T. J.    Kippenberg, and C. Koos, “Full C and L-Band Transmission at 20    Tbit/s Using Cavity-Soliton Kerr Frequency Combs,” In the    Proceedings of the 2015 Conference on Lasers and Electro-Optics    (CLEO 2015), Postdeadline Paper: jTh5C.8;-   8. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios, G.    Pakulski, D. Poitras, G. Z. Xiao, and Z. Y. Zhang, “Uniform    90-channel multiwavelength InAs/InGaAsP quantum dot laser,”    Electron. Lett., 43, 8, 458-460 (April 2007);-   9. Z. G. Lu, J. R. Liu, S. Raymond, P. J. Poole, P. J. Barrios,    and D. Poitras, “312-fs pulse generation from a passive C-band    InAs/InP quantum dot mode-locked laser,” Optics Express 16 (14),    10835-10840 (July 2008);-   10. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios,    and D. Poitras, “1.6-μm multiwavelength emission of an InAs/InGaAsP    quantum dot laser,” IEEE Photonics Technology Letters, 20, No .2,    pp. 81-83 (January 2008);-   11. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios,    and D. Poitras, “Dual-wavelength 92.5 GHz self-mode-locked InP-based    quantum dot laser,” Optics Letters, Vol. 33, No. 15, pp. 1702-1704    (August 2008).-   12. Z. G. Lu, J. R. Liu, P. J. Poole, S. Raymond, P. J. Barrios, D.    Poitras, G. Pakulski, P. Grant and D. Roy-Guay, “An L-band    monolithic InAs/InP quantum dot mode-locked laser with femtosecond    pulses,” Optics Express, Vol. 17, No. 16, pp.13609-13614 (August,    2009).-   13. Z. G. Lu, J. R. Liu, S. Raymond, P. J. Poole, P. J. Barrios,    and D. Poitras, “Quantum-dot-based semiconductor waveguide devices,”    U.S. Pat. No. 7,769,062B2 (3 Aug. 2010).-   14. Z. J. Jiao, J. R. Liu, Z. G. Lu, X. P. Zhang, P. J. Poole, P. J.    Barrios, and D. Poitras, “A C-Band InAs/InP Quantum Dot    Semiconductor Mode-Locked Laser Emitting 403-GHz Repetition Rate    Pulses,” IEEE Photonics Technol. Lett., vol. 23, pp. 543-545, 2011.-   15. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios,    and D. Poitras, “Multiband multiwavelength mode-locking lasers,”    U.S. Pat. No. 7,991,023B2, (2 Aug. 2011).-   16. Z. G. Lu, J. R. Liu, P. J. Poole, Z. J. Jiao, P. J. Barrios, D.    Poitras, J. Caballero, and X. P. Zhang, “Ultra-high repetition rate    InAs/InP quantum dot mode-locked lasers,” Optics Communications,    Vol. 284, No. 9, pp. 2323-2326 (May 2011).-   17. Z. G. Lu, J. R. Liu, P. J. Poole, P. J. Barrios, D.    Poitras, C. Y. Song, S. D. Chang, J. Weber, L. Mao, H. P. Ding, P.    Zhang, P. H. Ma, X. S. Tong, C. Flueraru, and S. Janz, “Coherence    comb laser sources: quantum dots, packaging and active control,”    (Invited paper), The 18th European Conference on Integrated Optics    2016, Warsaw, Poland, 17-21 May 2016.-   18. Z. G. Lu, J. R. Liu, P. J. Poole, C. Y. Song, J. Weber, L.    Mao, S. D. Chang, H. P. Ding, P. J. Barrios, D. Poitras and S. Janz,    “Integrated InAs/InP quantum dot coherent comb lasers,” (Invited    paper), SPIE Photonics West 2017, San Francisco, Calif., USA, 28    Jan.-2 Feb. 2017.-   19. Regan Watts, Ricardo Rosales, Francois Lelarge, Abderrahim    Ramdane, and Liam Barry, “Mode coherence measurements across a 1.5    THz spectral bandwidth of a passively mode-locked quantum dash    laser,” Optics Letters, Vol. 37, 1499-1501 (2012).-   20. T. Habruseva, S. O'Donoghue, N. Rebrova, F. Kéfélian, S. P.    Hegarty, and G. Huyet, “Optical linewidth of a passively mode-locked    semiconductor laser,” Optics Letters, Vol. 34, 3307-3309 (2009).-   21. Kristian Zanette, John Cartledge and Maurice O'Sullivan,    “Correlation properties of the phase noise between pairs of lines in    a quantum-dot optical frequency comb source,” In the Proceedings of    The Optical Fiber Communication Conference 2017, Th3l.-   22. Akram Akrout, Alexandre Shen, Romain Brenot, Frédéric Van Dijk,    Odile Legouezigou, Frederic Pommereau, Francois Lelarge, Abderrahim    Ramdane, and Guang-Hua Duan, “Separate Error-Free Transmission of    Eight Channels at 10 Gb/s Using Comb Generation in a    Quantum-Dash-Based Mode-Locked Laser,” IEEE Photonics Technol.    Lett., vol. 21, pp. 1746-1748 (2009).-   23. Yousra Ben M′Sallem, Quang Trung Le, Laurent Bramerie, Quoc-Thai    Nguyen, Eric Borgne, Pascal Besnard, Alexandre Shen, Francois    Lelarge, Sophie LaRochelle, Leslie A. Rusch, and Jean-Claude Simon,    “Quantum-Dash Mode-Locked Laser as a Source for 56-Gb/s DQPSK    Modulation in WDM Multicast Applications,” IEEE Photonics Technol.    Lett., vol. 23, pp. 453-455 (2011).-   24. Regan T. Watts, Stuart G. Murdoch, and Liam P. Barry, “Spectral    linewidth reduction of single-mode and mode-locked lasers using a    feed-forward heterodyne detection scheme,” In the Proceedings of the    2014 Conference on Lasers and Electro-Optics (CLEO 2014), Paper:    STh3O.8.-   25. Joerg Pfeifle, Regan Watts, Igor Shkarban, Stefan Wolf, Vidak    Vujicic, Pascal Landais, Nicolas Chimot, Siddharth Joshi, Kamel    Merghem, Cosimo Calò, Marc Weber, Abderrahim Ramdane, Francois    Lelarge, Liam P. Barry, Wolfgang Freude, and Christian Koos,    “Simultaneous Phase Noise Reduction of 30 Comb Lines from a    Quantum-Dash Mode-Locked Laser Diode Enabling Coherent Tbit/s Data    Transmission,” In the Proceedings of The Optical Fiber Communication    Conference 2015, Paper: Tu3I.5.-   26. P. Marin, J. Pfeifle, J. N. Kemal, S. Wolf, K. Vijayan, N.    Chimot, A. Martinez, A. Ramdane, F. Lelarge, W. Freude1, and C.    Koos1, “8.32 Tbit/s Coherent Transmission Using a Quantum-Dash    Mode-Locked Laser Diode,” In the Proceedings of the 2016 Conference    on Lasers and Electro-Optics (CLEO 2016), Paper: STh1F.1.-   27. P. J. Poole, R. L. Williams, J. Lefebvre and S. Moisa, “Using    As/P exchange processes to modify InAs/InP quantum dots”, J. Crystal    Growth, Vol. 257, pp. 89-96, 2003.-   28. P. J. Poole, K. Kaminska, P. Barrios, Z. G. Lu and J. R. Liu,    “Growth of InAs/InP-based quantum dots for 1.55 μm laser    applications,” J. Crystal Growth, Vol. 311, pp. 1482-1486, 2009.-   29. J. N. Kemal, P. Marin-Palomo, K. Merghem, G. Aubin, C. Calo, R.    Brenot, F. Lelarge, A. Ramdane, S. Randel, W. Freude, C. Koos,    “32QAM WDM Transmission Using a Quantum-Dash Passively Mode-Locked    Laser with Resonant Feedback” OFC 2017 @ OSA 2017 Th5C.3.-   30. K. Merghem, V. Panapakkam, Q. Gaimard, F. Lelarge, A. Ramdane,    “Narrow Linewidth Frequency Comb Source based on Self-injected    Quantum-Dash Passively Mode-Locked Laser” OFC 2017 @ OSA 2017    SW1C.5.

Other advantages that are inherent to the structure are obvious to oneskilled in the art. The embodiments are described herein illustrativelyand are not meant to limit the scope of the invention as claimed.Variations of the foregoing embodiments will be evident to a person ofordinary skill and are intended by the inventor to be encompassed by thefollowing claims.

The invention claimed is:
 1. A method for narrowing a linewidth of acoherent comb laser (CCL) comprising: providing a mode-lockedsemiconductor coherent comb laser (CCL) with a laser cavity defined byan active gain material in a waveguide between two facets, the CCLadapted to output of at least 4 mode-locked lines, each with an originallinewidth of less than 100 MHz; tapping a fraction of a power from theCCL from the laser cavity to form a tapped beam; propagating the tappedbeam to an attenuator to produce an attenuated beam and propagating theattenuated beam back to the laser cavity, on a solid waveguide; andreinserting the attenuated beam into the laser cavity, where thereinserted beam has a power less than 10% of a power of the tapped beam,where the reinsertion allows the CCL to be operated to output themode-locked lines, each with a linewidth of less than 80% of theoriginal linewidth, and an optical path between tapping and reinsertionis polarization maintaining.
 2. The method of claim 1 where the providedmode-locked semiconductor CCL: is electrically pumped; controlled by alow noise laser driver, and temperature controlled; is a ridge waveguidelaser with edge facets forming a Fabry-Perot cavity; is one of: a smalledge-emitting laser, an external cavity laser, a monolithic(internal-cavity) laser, a diode bar laser, a stacked diode bar laser, asurface-emitting laser (VCSEL), such as an optically pumpedsurface-emitting external-cavity semiconductor laser (VECSEL), or aquantum cascade laser; or has an active gain material comprising quantumwells, dots, dashes or rods formed of GaAs, AlGaAs, InGaAs, InAs,GaInNAs, GaN, GaP, InGaP, InP, GaInP, or a combination thereof.
 3. Themethod of claim 1 where an optical path length (OPL) of the combinedtapping, propagation and reinserting steps is between 5 and 50 m, andthe attenuation level is between 15 and 60 dB.
 4. The method of claim 1,where tapping the CCL comprises: collecting output of a backside facetof the CCL, or providing a coupler to tap a fraction of an output of theCCL.
 5. The method of claim 4 where reinserting the attenuated beamcomprises reinjecting the attenuated beam into the laser cavity via thebackside facet, or the coupler.
 6. The method of claim 1 wherepropagating the tapped beam to an attenuator comprises: one of: couplingthe tapped beam from a bidirectional waveguide path to a unidirectionalwaveguide circuit including the attenuator; and coupling the tapped beamfrom a bidirectional waveguide path, which includes the attenuator, to aunidirectional waveguide circuit; and further comprises one of:providing the attenuator on a bidirectional waveguide path that includesa reflector; and providing a partial reflector on the bidirectionalwaveguide path that serves to both attenuate and reflect the tappedbeam.
 7. The method of claim 1 wherein the attenuator is a variableoptical attenuator.
 8. The method of claim 1 where the attenuator has anattenuation range of at least 10 dB; avoids creating spuriousreflections; attenuates each of the lines to at least 10 dB; and doesnot vary an optical path length (OPL) of the combined tapping,propagation and reinserting steps while changing degree of attenuation.9. The method of claim 7 where the attenuator controls lighttransmission by an aperture variation, with partial occlusion of thebeam.
 10. The method of claim 1, where a solid waveguide is provided byPM single mode optical fibres, a microphotonic chip, a photonic crystalarrangement, or an integrated optical system.
 11. The method of claim 1where one of the mode-locked lines output has a stability such that overa one hour period, the linewidth does not vary by more than 100 kHz. 12.A narrow linewidth multi-wavelength laser (MWL) comprising: amode-locked semiconductor coherent comb laser (CCL) with a primary lasercavity defined by an active gain material in a waveguide between twofacets, the CCL adapted to output of at least 4 mode-locked lines, eachwith an original linewidth of less than 100 MHz; and a secondary lasercavity coupled to the primary laser cavity for tapping a beam of the CCLand propagating the tapped beam to an attenuator and reinserting theattenuated beam into the primary laser cavity at a power less than 10%of a power of the tapped beam, the secondary laser cavity consisting ofpolarization maintaining solid waveguides between polarizationmaintaining components, wherein a linewidth of each of the at least 4lines is reduced in proportion to a difference in optical path lengthbetween the secondary laser cavity and the primary laser cavity.
 13. TheMWL of claim 12 wherein the CCL: is electrically pumped; controlled by alow noise laser driver, and temperature controlled; is a ridge waveguidelaser with edge facets forming a Fabry-Perot cavity; is one of: a smalledge-emitting laser, an external cavity laser, a monolithic(internal-cavity) laser, a diode bar laser, a stacked diode bar laser, asurface-emitting laser (VCSEL), such as an optically pumpedsurface-emitting external-cavity semiconductor laser (VECSEL), or aquantum cascade laser; or has an active gain material comprising quantumwells, dots, dashes or rods formed of GaAs, AlGaAs, InGaAs, InAs,GaInNAs, GaN, GaP, InGaP, InP, GaInP, or a combination thereof.
 14. TheMWL of claim 12 where an optical path length of the secondary lasercavity is between 5 and 50 m, and the attenuation level is between 15and 60 dB.
 15. The MWL of claim 12 where the secondary laser cavitycomprises an optical coupling from one of a backside facet of the CCL,and/or a tap of an output of the CCL via which the beam is tapped and/orreinserted.
 16. The MWL of claim 12 where the secondary laser cavitycomprises: a bidirectional waveguide path coupled to a unidirectionalwaveguide circuit including the attenuator; the bidirectional waveguidepath including at least one of: a reflector; and a partial reflector onthe bidirectional waveguide path that serves to both attenuate andreflect the tapped beam.
 17. The MWL of claim 12 where the attenuator isa variable optical attenuator with an attenuation range of at least 10dB, provisioned to avoid creating spurious reflections, to attenuateeach of the lines to at least 10 dB, and to not vary an OPL of thesecondary laser cavity while changing the degree of attenuation.
 18. TheMWL of claim 12 where the attenuator controls light transmission by anaperture variation, with partial occlusion of the beam.
 19. The MWL ofclaim 12 where the secondary laser cavity comprises an optical pathbetween tapping and reinsertion provided by: single mode optical fibres;a free-space optical system; a microphotonic chip; a photonic crystalarrangement; or an integrated optical system.
 20. The MWL of claim 12where one of the at least 4 lines has a stability such that over a onehour period, the linewidth does not vary by more than 100 kHz.